1. Field of the Invention
This invention relates to processes for preparing isocyanates and more particularly to the process of preparing methyl isocyanate which is useful as starting material in the manufacture of e.g. N-methylcarbamino acid esters possessing a wide range of the activity against insects.
2. Description of the Prior Art
Isocyanates currently find extensive application in chemical industry. Development of processes for preparing isocyanates is in progress now, and is aimed at providing compositions and reaction conditions which offer higher yields.
Known in the art is a process for preparing methyl isocyanates (U.S. Pat. No. 3,644,461) by reacting phosgene with methyl amine or with chlorine hydrate of the same at 250.degree. C. giving rise to a formation of N-methyl carbamoyl chloride and decomposition thereof until methyl isocyanate is formed: ##STR1##
Gaseous methyl amine and phosgene are reacted at the first stage in a continuous fashion at a temperature range of 350.degree. to 400.degree. C. and atmospheric pressure. The reaction gases, which are represented by N-methyl carbamoyl chloride and HCl, are flown through a tower wherein carbon tetrachloride cooled to -10.degree. C. is sprayed to cool the reaction gases to 65.degree.-70.degree. C.
A solution of methyl carbamoyl chloride in carbon tetrachloride thus obtained is passed to a tower to strip unreacted phosgene. Next, hydrogen chloride is split off methyl carbamoyl chloride present in solution with carbon tetrachloride by methyl aniline at 45.degree. to 50.degree. C. to produce methyl isocyanate. Methyl isocyanate is prepared in two consecutive towers. The yield of methyl isocyanate is .apprxeq.90% of the theoretical. The methyl isocyanate obtained contains 98% of the basic substance and 2% carbon tetrachloride.
Though a rather high yield of methyl isocyanate is provided by this process, it still follows a multi-stage procedure, may be carried out with a complicated equipment, is based on costly and what is extremely important on highly toxic starting materials. Also, the hydrochloric acid produced in the process causes the equipment to corrode. Difficulties are also met with in regeneration of auxiliary substances involved in the process.
Thus, waste materials from the regeneration of dimethyl aniline and carbon tetrachloride need to be subjected to thermal decomposition with resulting environmental pollution.
Other prior art processes for preparing methyl isocyanate include such as by thermally decomposing N,N-diphenyl-N'-methylurea at 240.degree. to 249.degree. C. (Siefken, Ann. (1949) 562, 75) or phenyl-N-methyl carbamate at 240.degree. to 250.degree. C. (Japanese Patent Publication No. 1652-73).
Methyl isocyanate may also be prepared by reacting methyl amine with diphenyl carbonate at a temperature of 200.degree. C. but lower than the boiling point of diaryl carbonate (Japanese Patent Publication No. 20534-73) or by reacting N,N-dimethyl area with diaryl carbonate in a mole ratio of 1:1.5 to 1.4 and temperature of 200.degree.-300.degree. C. (Japanese Patent Publication No. 1652-73).
To practice these processes, however, there is need for scarce materials, which are produced in the processes involving phosgene and scarce amines.
U.S. Pat. No. 3,584,028 discloses a process for the manufacture of organic isocyanates having from C.sub.1 to C.sub.20 in the carbon group comprising reacting an alkali metal cyanate and/or alkaline earth metal cyanate with organic chlorides in an organic solvent in the presence of a catalyst, namely an alkali metal bromide or alkaline earth metal bromide, or alkali metal iodide, or alkaline earth metal iodide at elevated temperature.
To practice the process a reaction mixture conjointly contains 0.5 to 5 moles, preferably 0.8 to 1 mole of the cyanates, 0.01 to 10 moles, preferably 0.5 to 0.25 mole of the catalyst, 10 to 100 moles, preferably 25 to 50 moles of the organic solvent, per 1 mole of the organic chloride. The reaction temperature is 25.degree. to 300.degree. C., preferably 50.degree. to 150.degree. C., pressure may be from below 1 to 703 kg/cm.sup.2, and the time of contacting equals 50 min.
Examples show benzyl chloride, allyl chloride, N-octyl chloride as the organic chlorides.
The last specified process does not involve highly toxic substances. But the yield of isocyanates is from 6.8 to 55%, the remainder 45% or more being isocyanates. The time (50 min.) of contacting of the cyanates with halides, due to which isocyanate obtained is polymerized is a reason, among others equally important, for the formation of isocyanurates.
This process is also disclosed as suitable for preparing isocyanate having 1 carbon atom, i.e. methyl isocyanate.
In this instance methyl chloride is used as organic chloride.
Since methyl isocyanate among alkyl isocyanates is the most sensitive to heating and to the presence of salts the cations of which being the alkali metals and the alkaline earth metals, while anions being halogenides, OCN', CO".sub.3, etc., the yield of methyl isocyanate is expected to be still lower.